Cost-effective storage for the electrical grid has been sought from the beginning of electrical service delivery but is not yet available. The variation in power demand throughout a day, and season-to-season requires having generating stations that are utilized only part of the year, increasing capital and operations and maintenance costs for stations used at less than full capacity. Furthermore, some generating stations are difficult to throttle or shut down and return to full power within short periods of time. This lack of practicable energy storage results in the vast majority of the challenges faced by parties operating electrical grid(s).
With the increased recognition that continued carbon emissions from burning fossil fuels is unsustainable on multiple levels, and that proliferation concerns exist for nuclear power, it has become clear that relatively large amounts of renewable energy (RE) will be needed to provide power for the grid. Hydroelectric power, when combined with a reservoir, is one RE source that can be throttled up and down to match the varying power loads, also called “load-following”. Geothermal and Ocean Thermal Energy Conversion are also good baseload RE resources, despite their limited locations. However, the solar wind, wave, tidal, and current energies are all intermittent. Energy storage is required for those sources to substantially contribute to the grid energy supply.
Cost parameters of several leading storage technologies may be considered for large scale energy systems. Each technology has its own cost drivers. Pumped hydroelectric, for example, has been used for many decades and is often considered the standard by which other grid-energy-storage ideas are judged. It is efficient, consumes no fuel upon harvesting the stored energy, but is constrained by geography. A substantial elevation change and two reservoirs are typically required. Most of the viable sites in North America are considered to be already developed. Regardless of cost, it does not appear that pumped hydroelectric will be able to contribute much additional energy storage capacity. It is also fairly expensive in terms of capital cost per unit power ($/kW) but nonetheless is widely used when available because of the fairly low capital cost per unit energy ($/kWh).
Considerable effort is going into “conventional” batteries, but most of that effort is focused on electric vehicle energy storage, where weight is a critical parameter. As such, many of the “new” battery technologies are actually considerably more expensive than can be tolerated for grid-energy-storage systems. Thus, these batteries for hybrid electric vehicles are often able to provide considerable power per unit cost, but are still very expensive per unit energy. Flow batteries are a newer technology where the chemicals are stored in tanks and reacted in systems similar to fuel cells. The cost of the fairly unusual chemicals used as the reactants leads to moderately high cost per unit energy and unit power.
Compressed Air Energy Storage (CAES) is an attractive energy storage technology that overcomes many drawbacks of known energy storage technologies. The conventional approach for CAES is to use a compressor to store the compressed air underground. The energy is harvested by expanding the compressed air through a turbine. In this process, the air is mixed with natural gas, combusted and expanded through the turbine. The system operates at high pressure in order to take advantage of the modest volume of the underground cavern or aquifer. The result is a system that operates with constant volume and variable pressure during the storage and retrieval process, which results in extra costs for the compressor and turbine system, since they operate best at a single design pressure. The heating during compression and the cooling during expansion of air also require special attention in order to obtain suitable efficiencies.
Conventional CAES reheats the air efficiently using combustion of natural gas (often by absorbing heat from the gas turbine exhaust). Such systems often have two separate compressors and turbines. They therefore have a greater capital expense, over and above the cost of the natural gas. The result is that the power plant, when utilizing purchased off-peak power to charge the air reservoir, generates power with about ½ the use of natural gas per unit energy but with a moderately expensive set of equipment and higher fuel costs.
A need exists to provide grid-scale energy storage that is more energy-efficient, lower in cost, more responsive, and more geographically ubiquitous than traditional underground CAES.